An electronic control unit mounted to a vehicle, such as a body control ECU which controls electric components of a vehicle, transitions to a sleep state when an ignition key is OFF, a standby state. In the sleep state, functions of the electronic control unit are stopped to a maximum extent to minimize the power consumption of a battery. The electronic control unit has a function of, during the sleep, keeping monitoring door unlock, turn on of the ignition key and the like to promptly respond to an occupant's operation.
A commonly known method of monitoring a vehicle state includes intermittently waking up a microcomputer of the electronic control unit, retrieving a signal from a sensor detecting a state of a monitor target, and determining whether or not a change in this state occurs based on the retrieved signal (see Patent Literature 1 for example). In order to reduce the power consumption while maintaining determination accuracy of the monitor target state, this method of Patent Literature 1 applies A/D sampling to an analog signal from the sensor when the microcomputer is intermittently waked up, and determines whether or not an obtained digital signal exceeds a predetermined threshold. When the digital signal exceeds the threshold, a wakeup cycle of the microcomputer is shortened and the occurrence and non-occurrence of the change in the monitor target state are determined.